1. Field of the Invention
This invention relates to a scanning optical system of the so-called flying spot type which scans a surface to be scanned by a beam spot or a scanning optical system of the so-called flying image type in which the light beam from each portion of a surface to be scanned is received by light-receiving means through a deflector to thereby scan the surface to be scanned.
2. Description of the Prior Art
The above-described scanning optical system is utilized in a recording apparatus for recording electrical signals to be recorded or a reading apparatus for reading the information of a surface to be scanned.
In the scanning optical system, filters are used for various purposes and these filters are usually disposed at such positions whereat the light beam in the scanning optical system is not laterally moved. However, for example, in a laser beam printer adopting a scanning optical system of the flying spot type, if a filter is disposed between a light source unit and movable deflecting means, the laser beam always passes through the same position of the filter and therefore, the filter is deteriorated, and this has sometimes led to the spoiled original function of the filter. Also, where an imaging lens 6 for scanning is present between the deflecting means and the surface to be scanned, there occurs a result that the intensity of the beam spot differs between the beam passing through the central portion of the lens 6 and the beam passing through the marginal portion of the lens 6 due to the cos fourth power rule of the lens. In such a case when it is necessary to scan the surface to be scanned by beam spots of the same intensity, it is desirable that a filter having a lower light transmission factor in the central portion thereof than in the opposite end portions be disposed in the optical path between the movable deflecting means and the surface to be scanned. Also, in a scanning optical system of the flying image type, where it is desired to detect the light beams from various portions of the surface to be scanned at equal quantities of light by light-receiving means, it sometimes becomes necessary to dispose the filter as described above between the surface to be scanned and the deflecting means.
The filter heretofore used has been of a straight planar shape and it has been found that the inconveniences as described below occur if such a filter is disposed between the movable deflecting means and the surface to be scanned.
Referring to FIG. 1 of the accompanying drawings, the light beam 1 from a laser light source or the like enters a polygonal mirror 2 being rotated in the direction of arrow, is reflected thereby and becomes a scanning light 3. The scanning light 3 passes throught a straight planar filter 4 and scans a surface 5 to be scanned.
The amount of absorption of light of the filter is proportional to the length of the optical path in the filter. In FIG. 1, the length of the optical path in the filter of the scanning light 3a entering the filter perpendicularly thereto is t if the thickness of the filter is t. The length l of the optical path in the filter of the scanning light 3b entering at an angle of incidence .alpha. is ##EQU1## and since t&lt;l, the scanning light 3b on the surface 5 to be scanned is smaller in quantity of light than the scanning light 3a. Accordingly, as the angle of incidence .alpha. becomes smaller, the quantity of transmitted light becomes smaller and thus, irregularity of quantity of light occurs in the scanning area. In the flying image scanning optical system, this appears as the irregularity of the quantity of light entering the light-receiving element.
Also, as shown in FIG. 2 of the accompanying drawings, the scanning light 3 having entered the filter 4 at the position a thereof passes via a route of a.fwdarw.b.fwdarw.c and part of this light is reflected by the inner surface b of the filter 4 and some of the light is reflected via b.fwdarw.d.fwdarw.e.fwdarw.f. Where the scanning light is monochromatic light such as a laser beam, interference of light is caused by the reflected light a.fwdarw.b.fwdarw.d.fwdarw.e.fwdarw.f and the transmitted light d.fwdarw.e.fwdarw.f. That is, when the difference in length of the optical path between these lights is even number times the half-wavelength of the monochromatic light which is the scanning lingt, these lights strengthen each other and, when said difference in length of the optical path is odd number times the half-wavelength of the monochromatic light, these lights weaken each other. Therefore, a fringe pattern representing the strength and weakness of the quantity of light is created in the scanning area. This irregularity of quantity of light becomes denser and more conspicuous as the scanning angle is greater.